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Biomedical subjects

S Chang

Publications and source records attributed to S Chang.

At least 181 records · Page 10Linked to original sources
PubMed1996

Long-acting growth hormones produced by conjugation with polyethylene glycol.

Derivatives of human growth hormone (hGH) of increasing size were produced by reaction with the N-hydroxysuccinimide ester of polyethylene glycol-5000 (PEG5000), a 5-kDa reagent that selectively conjugates to primary amines. By adjusting the reaction conditions and purification procedure, it was possible to isolate hGH derivatives containing up to seven PEG moieties that altered the Stokes radius and thereby the effective molecular masses of the unmodified hormone from 22 to 300 kDa. Fortunately, the most reactive amines were ones that did not lie in either of the two sites important for receptor binding. Nonetheless, increasing the level of PEG modification linearly reduced the affinity of hGH for its receptor and increased the EC50 in a cell-based assay up to 1500-fold. Most of the reduction in affinity was the result of slowing the association rate for the receptor. The clearance rate of hGH in rats was inversely proportional to effective molecular weight and closely fit a filtration model. We have tested the potency of these analogs by injecting them daily or every 6 days into hypophysectomized rats and determining the effects on body and organ growth. The efficacy of these analogs was optimal for hGH conjugated with 5 eq of PEG5000, and the potency was increased by about 10-fold compared with unmodified hGH. Such PEG-hGH derivatives show promise as long-acting alternatives to daily injections of hGH. More generally these studies show that improving hormone clearance properties, even at the expense of reducing receptor binding affinity, can lead to dramatic increases in hormone efficacy.

Animals↗
PubMed1996

Involvement of integrins alpha v beta 3 and alpha v beta 5 in ocular neovascular diseases.

Angiogenesis underlies the majority of eye diseases that result in catastrophic loss of vision. Recent evidence has implicated the integrins alpha v beta 3 and alpha v beta 5 in the angiogenic process. We examined the expression of alpha v beta 3 and alpha v beta 5 in neovascular ocular tissue from patients with subretinal neovascularization from age-related macular degeneration or the presumed ocular histoplasmosis syndrome or retinal neovascularization from proliferative diabetic retinopathy (PDR). Only alpha v beta 3 was observed on blood vessels in ocular tissues with active neovascularization from patients with age-related macular degeneration or presumed ocular histoplasmosis, whereas both alpha v beta 3 and alpha v beta 5 were present on vascular cells in tissues from patients with PDR. Since we observed both integrins on vascular cells from tissues of patients with retinal neovascularization from PDR, we examined the effects of a systemically administered cyclic peptide antagonist of alpha v beta 3 and alpha v beta 5 on retinal angiogenesis in a murine model. This antagonist specifically blocked new blood vessel formation with no effect on established vessels. These results not only reinforce the concept that retinal and subretinal neovascular diseases are distinct pathological processes, but that antagonists of alpha v beta 3 and/or alpha v beta 5 may be effective in treating individuals with blinding eye disease associated with angiogenesis.

Animals↗
PubMed1996

The role of lipoprotein lipase and apoprotein E in the recognition of chylomicrons and chylomicron remnants by cultured isolated mouse hepatocytes.

Lipoprotein lipase (LPL) has been proposed to play a role in the uptake of chylomicron remnants by hepatocytes by mediating the binding of these lipoproteins to cell-surface glycosaminoglycans and to the low-density-lipoprotein receptor-related protein (LRP). This proposal is based on studies that examined the binding of chylomicrons to HepG2 cells, fibroblasts and Chinese hamster ovary cells in culture, in the presence of large amounts of LPL [Beisiegel (1995) Curr. Opin. Lipidol. 6, 117-122]. We have investigated whether LPL attached to the surface of chylomicrons enhances the binding and uptake of these lipoproteins to isolated hepatocytes maintained in culture. Bovine milk LPL was bound to mouse chylomicrons, double-labelled in vivo with [3H]retinol (in retinyl esters) and with [14C]palmitic acid (in triacylglycerols), collected from the mesenteric lymph of normal mice and from mice lacking the apoprotein E (apo E) gene. Normal chylomicrons (containing apo E) and apo E-free chylomicrons, with or without bound LPL, were incubated with cultured hepatocytes isolated from mice lacking the apo E gene. At 0 degree C LPL did not enhance the binding of the normal or apo E-free chylomicrons by the hepatocytes. When incubations were performed at 37 degrees C the triacylglycerols of normal and apo E-free chylomicrons were hydrolysed by LPL and there was a significant uptake of [14C]fatty acids and [3H]retinol by the hepatocytes. The addition of heparin or lactoferrin, a known inhibitor of hepatic uptake of chylomicron remnants, to the incubation medium inhibited the uptake of [3H]retinol, present in the lipoprotein core, but not the uptake of the [14C]fatty acids. We conclude that: (1) LPL attached to chylomicrons in amounts sufficient to effectively hydrolyse their core triacylglycerols does not enhance the binding of these lipoproteins to the surface of isolated hepatocytes; (2) the recognition and uptake of chylomicrons by hepatocytes requires that these lipoproteins be first hydrolysed by LPL; and (3) the uptake of lipolysed chylomicrons (remnants) by hepatocytes does not require the mediation of apo E.

Animals↗
PubMed1996

Erratum to Mutation spectrum of 2-chloroethyl methanesulfonate in Drosophila melanogaster premeiotic germ cells' [Mutation Res. 331 (1995) 213-224].

The 2-chloroethyl methanesulfonate (2CIEMS)-induced alcohol dehydrogenase (Adh) null germline mutation frequency in treated Drosophila melanogaster second instar larval gonia was two orders of magnitude greater than the spontaneous mutation frequency. DNA sequence analysis of 83 Adh null mutations showed that 40 mutations of independent origin were at 23 sites in the Adh gene. The mutation spectrum contained only GC --> AT transitions with 35 mutations (87.5%) at the middle or 3' guanine. In addition, characteristics of glutathione (GSH)-mediated bioactivation were determined for 2CIEMS in vitro. Rates of GSH-mediated conjugation, catalyzed by purified rat liver glutathione-S-transferase (GST), and binding of [35S]GSH-mediated conjugation products to calf thymus DNA were determined for 2CIEMS, 1,2-dichloroethane (EDC) and 1,2-dibromoethane (EDB). The relative rates of GSH-mediated conjugation were the following: 5 mM EDB > 40 mM 2CIEMS > 40 mM EDC. A similar trend was observed for DNA binding of the [35S]GSH-mediated conjugation products when differences in mutagen concentration were considered: EDB > 2CIEMS > EDC. The ratios of DNA binding to GSH conjugation calculated for EDB, EDC and 2CIEMS were 6.8 x 10(-5), 9.3 x 10(-5) and 19.1 x 10(-5), respectively. A narrow range, less than a 3-fold difference, in the ratios of DNA binding to GSH conjugation indicates that the bioactivation of 2CIEMS is mediated by the same mechanism as EDB and EDC. Consequently, 2CIEMS, EDC and EDB may induce a specific mutation in premeiotic germ cells.

Animals↗
PubMed1996

Disruption of the MacMARCKS gene prevents cranial neural tube closure and results in anencephaly.

MacMARCKS is a member of the MARCKS family of protein kinase C (PKC) substrates. Biochemical evidence demonstrates that these proteins integrate calcium and PKC-dependent signals to regulate actin structure at the membrane. We report here that deletion of the MacMARCKS gene prevents cranial neural tube closure in the developing brain, resulting in anencephaly. This suggests a central role for MacMARCKS and the PKC signal transduction pathway in the folding of the anterior neural plate during the early phases of brain formation, and supports the hypothesis that actin-based motility directs cranial neural tube closure.

Anencephaly↗
PubMed1996

Mechanisms of drug-induced lupus. II. T cells overexpressing lymphocyte function-associated antigen 1 become autoreactive and cause a lupuslike disease in syngeneic mice.

Current theories propose that systemic lupus erythematosus develops when genetically predisposed individuals are exposed to certain environmental agents, although how these agents trigger lupus is uncertain. Some of these agents, such as procainamide, hydralazine, and UV-light inhibit T cell DNA methylation, increase lymphocyte function-associated antigen 1 (LFA-1) (CD11a/CD18) expression, and induce autoreactivity in vitro, and adoptive transfer of T cells that are made autoreactive by this mechanism causes a lupuslike disease. The mechanism by which these cells cause autoimmunity is unknown. In this report, we present evidence that LFA-1 overexpression is sufficient to induce autoimmunity. LFA-1 overexpression was induced on cloned murine Th2 cells by transfection, resulting in autoreactivity. Adoptive transfer of the transfected, autoreactive cells into syngeneic recipients caused a lupuslike disease with anti-DNA antibodies, an immune complex glomerulonephritis and pulmonary alveolitis, similar to that caused by cells treated with procainamide. These results indicate that agents or events which modify T cell DNA methylation may induce autoimmunity by causing T cell LFA-1 overexpression. Since T cells from patients with active lupus have hypomethylated DNA and overexpressed LFA-1, this mechanism could be important in the development of human autoimmunity.

Animals↗
PubMed1996

In vitro fibroplasia: matrix contraction, cell growth, and collagen production of fibroblasts cultured in fibrin gels.

Extracellular matrix (ECM) reorganization, cell growth, and collagen synthesis/deposition are key features of fibroplasia during tissue repair. An in vitro fibrin gel culture model system simulating fibroplasia of wound repair was characterized. In the model system, fibrin gels were stabilized on plastic culture plates as hemispheres. In this way, fibroblasts were able to reorganize fibrin fibrils, resulting in a measurable decrease in gel thickness with no change in gel diameter, thereby producing a matrix with tension relevant to that of a repairing tissue. Within the study period, human dermal fibroblasts exhibited dynamic activities in cell growth and in reorganization and remodeling of the fibrin matrix. In the first 2 days of culture, fibroblasts quickly reorganized the fibrin matrix to 10% of its original thickness. Fibroblast proliferation occurred at a much slower rate compared to monolayer cultures. Proliferation continued at the same rate throughout the study in contrast to monolayer cultures, which ceased proliferation at confluence. Collagen synthesis was detected as early as the second day in culture. Type I collagen was the major collagen synthesized by fibroblasts with small amounts of type V and type III collagen. Collagen from either monolayer or fibrin gel cultures appeared identical when analyzed by two-dimensional peptide mapping of their CNBr fragments. Although collagen was detected biochemically from Day 2, organized collagen fibrils were apparently only in the later stage of cultures in transmission electron micrographs. Also, at this time, fibrin fibrils were largely removed and the matrix was filled with collagen fibrils and other filamentous ECM. The growth factor TGF-beta stimulated both fibrin gel contraction and collagen synthesis by fibroblasts. Therefore, using the model system, we have demonstrated that fibroblasts can actively reorganize the fibrin matrix and subsequently remodel it into a collagen-containing scar-like tissue. The unique features of this model system allow for creative designs in studying the complex mechanisms underlying tissue repair.

Cell Culture Techniques↗
PubMed1996

Stimulus-dependent phosphorylation of MacMARCKS, a protein kinase C substrate, in nerve termini and PC12 cells.

MacMARCKS (also known as myristoylated alanine-rich C kinase substrate (MARCKS)-related protein) is a member of the MARCKS family of protein kinase C substrates, which binds Ca2+/calmodulin in a phosphorylation-dependent manner. Immunoprecipitation demonstrated that MacMARCKS is present in both PC12 cells and in neurons. Upon depolarization of PC12 cells with 60 mM KCl, MacMARCKS phosphorylation increased 4-fold over basal levels in a Ca(2+)-dependent manner. By immunofluorescence microscopy, MacMARCKS was colocalized in PC12 cells to neurite tips with the synaptic vesicle membrane protein synaptophysin and to vesicles in the perinuclear region. Subcellular fractionation demonstrated that MacMARCKS associates tightly with membranes in PC12 cells. In Percoll-purified rat cerebrocortical synaptosomes, depolarization with 60 mM KCl in the presence of exogenous Ca2+ transiently increased MacMARCKS phosphorylation, whereas phorbol ester promoted a sustained increase in MacMARCKS phosphorylation. Subcellular fractionation of rat brain indicated that MacMARCKS was present in both soluble and particulate fractions; particulate MacMARCKS was associated with both small vesicles and highly purified synaptic vesicles. These results are consistent with a role for MacMARCKS in integrating Ca(2+)-calmodulin and protein kinase C-dependent signals in the regulation of neurosecretion.

Animals↗
PubMed1996

The distribution of collapsin-1 mRNA in the developing chick nervous system.

Collapsin-1 is a secreted glycoprotein that inhibits the extension of specific growth cones in vitro. It has been hypothesized to serve as a repulsive guidance cue for extending growth cones in vivo. Here we report the distribution of collapsin-1 message as demonstrated by in situ hybridization using digoxigenin-labeled RNA probes in wholemounts and tissue sections. In the early chick brain collapsin-1 is expressed in specific regions of the retina, the olfactory bulb, and the diencephalon. In the hindbrain collapsin-1 is first expressed in rhombomere 5 and later in bilaterally symmetric rostrocaudal stripes. Collapsin-1 is expressed in high levels in the ventral horn of the spinal cord and in ventricular stripes that extend rostrally to the hindbrain. In the periphery, collapsin-1 is expressed in the dermamyotome and in ectoderm and epidermis. Based on collapsin's expression patterns we tested axons extending from explants of ventral spinal cord and olfactory bulb for sensitivity to collapsin and show that the former are sensitive to collapsin whereas the latter are not. The distribution of collapsin mRNA is consistent with it playing a role in the organization of sensory axonal projections within the spinal cord and skin.

Animals↗
PubMed1996

The floating population: an informal process of urbanisation in China.

"This paper attempts to employ substantial source materials to shed light on the characteristics of population migration in a centrally planned society that is in the process of transforming to a market economy. In order to compare rural-urban migration in China with that in other developing countries, the data on 1985-1990 interprovincial population migration have been analysed. Some demographic research techniques on population migration, such as the impact on the place of origin and the place of destination, have been employed to assess the influence of rural-urban migration on China's long-term economic development. Economic betterment appears to be the sole motivation of rural population migration in China."

Asia↗